Resonant structures, used in electromagnetic filters and the like, often consist of a block or puck of dielectric material coated or plated with a conductor such as silver. One or more recesses or passageways are placed into the dielectric material, and the surfaces of the recesses or passageways are coated with a metal to form one or more center conductors. By adjusting the size, shape and configuration of the dielectric and center conductors, the properties of the resonant structure can be altered to obtain the desired electromagnetic characteristics.
Recently, high-temperature superconductors have been studied as materials for the coating on the center conductor or outer conductor of dielectric resonators. High-temperature superconductors, when cooled below their critical temperatures, have almost no electrical resistance and therefore result in extremely low losses in resonant structures containing them. Unfortunately, coating a superconductor onto a dielectric material is significantly more difficult than application of metals to those same dielectrics. Certain high-quality dielectrics such as sapphire exhibit excellent electromagnetic properties alone, but chemically react with high-temperature superconductors when coated with the superconductor. That reaction creates an undesirable microstructure in the high-temperature superconductor at the interface between the superconductor and the dielectric. Those undesirable microstructures may have poor electromagnetic properties (such as increased electrical resistance) and interfere with the overall quality of the resonant element. The effect of such electrical interference is magnified because the properties of the conductor or superconductor in the area where it contacts the dielectric are critical for optimum resonator performance. In addition, processing of the superconductor coating usually involves heating, which could damage a dielectric element if the dielectric is submitted to the processing along with the coating.